Silicon controlled rectifiers (SCR) are essentially two transistors, a PNP transistor and an NPN transistor interconnected to form a four layer PNP device. In the off state, the SCR presents an extremely high impedance to the flow of current therethrough. Several mechanisms exists by which an SCR can be turned on. Most commonly, current is applied to the base region of one of the two transistors making up the SCR by means of a voltage applied across the emitter and base of this transistor. However, because a pn junction has capacitance, the application of a rapid voltage change across the anode and cathode of the SCR creates a charging current which flows through the device from the anode to the cathode. When this charging current exceeds the value necessary to increase the common emitter current gains of the two transistors making up the SCR to a large enough value to turn the SCR on, the SCR turns on independently of the current in either of the gate electrodes. This turning on of the SCR in response to the rapid voltage change thereacross is undesirable when the SCR is designed to be controlled solely by a gate current.
In the prior art, many gate control circuits have been designed whereby the sensitivity of the SCR is greatly reduced during the application of a voltage thereacross. Once the supply voltage reaches a steady state across the SCR and the danger of a premature turn-on has passed, the gate control circuitry turns off to raise the sensitivity of the SCR and allow the application of a firing pulse thereto. In every instance the same circuitry is used to alter the sensitivity of the SCR and to fire the SCR at the desired time. This greatly limits the flexibility of the circuitry which can be used in conjunction with the SCR for altering the sensitivity and for firing the SCR at the desired time. In many instances this inflexibility, or inability to mix different types of semiconductor circuitry (e.g., NPN transistors and PNP transistors) greatly limits the usefulness of an SCR.